1. Field of the Invention
The present inventions relate to a process, by a combination of specific steps and a specific catalyst at a specific ratio, for hydrodesulfurization of a sulfur-containing gas oil with a boiling point of 220 to 380.degree. C. and an apparatus useful therefor.
2. Description of the Related Art
The straight run diesel gas oil obtained by distilling crude oil or the decomposition diesel gas oil obtained by decomposing heavy oil contains sulfur compounds, and the amount is in a range of 1 to 3 wt % as sulfur. When the diesel gas oil containing sulfur compounds is used as a diesel fuel, sulfur compounds will be exhausted in atmosphere as SO.sub.x and the environment will be polluted.
Therefore, these diesel gas oils are used as a fuel usually after being dehydrosulfurized to remove sulfur compounds. It is stated that the permissible value for amount of sulfur included in a diesel fuel is 0.05 wt % or less in JIS (Japanese Industrial Standard), and large-scale desulfurization arrangements have been constructed to achieve this value and are used. In addition, it is said that it is necessary to further decrease the amount of sulfur with a view to installing a purification catalyst, which reduces NOx in the automotive exhaust gas, into a diesel car in the future and using a part of the automotive exhaust gas again by circulating as a part of a diesel gas. This system is called an EGR system (EGR: Exhaust Gas Recirculation).
A catalyst, which consists essentially of cobalt or nickel, and molybdenum supported on an alumina carrier, has conventionally been used for the desulfurization of diesel gas oil so far. However, conventional catalysts have problems in that they can hardly desulfurize 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene, and therefore, in order to lower the sulfur content of the light oil product to the level of 0.05 wt % or less, it is necessary to raise the reaction temperature and the reaction pressure to a very high level, so that the construction cost of the arrangement, and the running costs, increase.
As for a process for improving the desulfurization activity for sulfur compounds to be hardly desulfurized, a catalyst whose carrier contains phosphorous and boron was reported in Japanese Unexamined Patent Publication (Kokai) No. 52-13503, and a catalyst to whose carrier zeolite was added was reported in Japanese Unexamined Patent Publication (Kokai) No. 7-197039. These catalysts have Br.o slashed.nsted acid sites and, thus, exhibit a high ability to isomerize a methyl group of dimethyldibenzothiophene and to hydrogenate a phenyl group, and high activity to desulfurize 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene.
However, the catalysts whose carriers contain phosphorous, boron or zeolite have drawbacks in that their desulfurization activities for alkylbenzothiophenes and dibenzothiophenes without 4- or 6-alkyl substituent, such as dibenzothiophene, and 1-, 2- or 3-methyldibenzothiophene are inferior to those of conventional catalysts consisting essentially of cobalt and molybdenum on an alumina carrier (F. van Looij et al. Applied Catalysis A: General 170, 1-12 (1998)). Moreover, said catalysts have further drawbacks in that, as they have Br.o slashed.nsted acid sites, they may easily cause a coloring of the eight oil product and, when they are used for an olefin-containing feedstock oil or are used at a high temperature of 350.degree. C. or higher, thiols and sulfides are occasionally generated to decrease the desulfurization ratio. In addition, they have another problem in that olefin elements in a feedstock may be polymerized at Br.o slashed.nsted acid sites to generate coke and the deactivation of catalyst may be accelerated. Even if olefins were not included in a feedstock, if sulfur compounds were desulfurized with said catalysts, olefins would be generated in situ, and it would cause an extraction of coke. This can be understood from the view that a coking speed when thiophene flows into said catalyst reaches ten times the coking speed reached when olefins or aromatic compounds flow into the catalyst (Catalysis Review, 24, (3), 343 (1982)).
It is difficult to desulfurize the sulfur compounds to a level of 0.05 wt % or less as sulfur even if any above-mentioned catalyst was used and studies have been carried out to deeply desulfurize a diesel gas oil from an aspect of process or reaction apparatus. For example, a process, containing two different steps processed under different reaction condition, which can deeply desufurize a diesel gas oil without any worsening of hue is proposed in Japanese Unexamined Patent Publication (Kokai) No. 7-102266. A deep hydrodesulfurization process, where a diesel gas oil is separated by distillation into a light fraction to be easily desulfurized and a heavy fraction to be hardly desulfurized and then those fractions after hydrodesulfurized individually are mixed into a deep desulfurized diesel gas oil product, is proposed in Japanese Unexamined Patent Publication (Kokai) No. 5-311179. However, said deep hydrodesulfurization process containing 2 different steps under different reaction conditions, which can desulfurize a diesel gas oil deeply without any worsening of hue, has an effect to improve a diesel gas oil hue but can hardly improve a further deep desulfurization. Said deep hydrodesulfurization process, where a gas oil feedstock is separated by distillation into a light fraction to be easily desulfurized and a heavy fraction to be hardly desulfurized and then those fractions after individually hydrodesulfurized are mixed into a deep desulfurized diesel gas oil product, has many problems in that a high reaction temperature and a high reaction pressure are needed for a heavy fraction to be hardly desulfurized.
Thus, these prior arts have many problems and they do not achieve an effective manufacturing of excellent diesel gas oil with low sulfur content when used for deep hydrodesulfurization of diesel gas oil as they are.